1. Field of the Invention
The present invention relates to a method of operating a semiconductor laser as a mode-locked semiconductor laser and to devices for implementing the method.
2. Background Information
A semiconductor laser is disclosed, for example, in Electronics Letters 26 (1990), No. 4, pages 243-244. The semiconductor laser described there is monolithically integrated on an n-doped indium phosphide substrate. It includes a cavity that is disposed on the substrate above a plane that is coplanar to the base face of the substrate and forms a branched, linearly contiguous structure in that plane. The underside of the substrate is called the base surface. Above the indium phosphide substrate, there extends a likewise n-doped buffer layer of indium phosphide. The cavity composed of indium gallium arsenide phosphide (InGaAsP) is disposed on the planar surface of this buffer layer. The cavity is considered to be a "linearly contiguous" region in the sense of a topological definition since it is composed of a single piece having a single edge. In this case, the cavity has the shape of a "Y". Above the cavity further layers are provided. Together with the cavity they form a "Y" shaped mesa structure formed by etching above the buffer layer.
In addition, the phenomenon of mode locking is also known per se. Mode locking is understood to be a resonance-like phenomenon in which a semiconductor laser is modulated with an alternating current of a frequency that corresponds to the reciprocal of the round-trip time of a light pulse in the semiconductor laser. The active mode locking of semiconductor lasers is a technique which permits the generation of light pulses of very short duration. Such light pulses are significant for communication transmission systems operating in the gigahertz range. Mode-locked semiconductor lasers make it possible to transmit data at speeds which lie noticeably above the highest frequencies at which semiconductor lasers could be modulated directly according to prior art techniques.
For the transmission of communications, light pulses or trains of light pulses are delayed by different times in delay lines, are then modulated in modulators and combined into a multiplex frame. The light pulses of such semiconductor lasers can also be employed for measuring particularly if frequencies are to be measured that lie above 20 GHz. Moreover, such semiconductor lasers serve to actuate optical correlators for coding the addresses of individual bits or for coding bit groups (header coding).
The publication Appl. Phys. Lett. 57 (1990), pages 759-761, discloses an actively mode-locked semiconductor laser which is provided with a simple, unbranched cavity for propagation of the light pulses generated within it. The cavity is composed of layers composed alternatingly of indium gallium arsenide phosphide and indium gallium arsenide (that is, a quantum well layer sequence). The cavity is divided into five regions; two regions adjacent the resonator end faces serve as modulators through which flows an alternating current; one region in the middle of the cavity serves as a saturable absorber; and two regions between the modulators and the absorber serve as active waveguides.